1. Field of the Invention
The present invention relates to a motor, and more particularly to, an apparatus for compensating for a speed error of a motor.
2. Description of the Conventional Art
In general, load characteristics of a compressor driven by a motor are varied by a compression type. Especially, according to the load characteristics of the compressor, when a single rotary compressor having a drive torque ripple is rotated, a torque ripple of the compressor increases, and a speed ripple of the motor increases due to the torque ripple of the compressor. That is, when a load having a torque ripple component such as the single rotary compressor is coupled to the motor, the speed ripple is generated by the torque ripple of the load.
FIG. 1 is a schematic diagram illustrating a conventional single rotary compressor. A compression unit (refer to FIG. 2) of the single rotary compressor is coupled to a rotor of a motor through a shaft, and a vane is coupled to the shaft of the motor. The operational principle of the compression unit will now be explained with reference to FIG. 2.
FIG. 2 is a diagram showing the operational principle of the compression unit.
Referring to FIG. 2, when the motor coupled to the compression unit is rotated, the vane of the compressor is rotated, to perform a compression stroke. When the vane of the compression unit is mechanically rotated once, the compression stroke is also performed once. Here, the motor coupled to the compressor has a speed ripple due to compression characteristics of the compressor, which will now be described with reference to FIGS. 3A and 3B.
FIGS. 3A and 3B are graphs showing characteristics of the conventional single rotary compressor.
As shown in FIGS. 3A and 3B, a torque ripple of the compressor is generated due to the compression characteristics (load characteristics) of the compressor, and a speed ripple is generated in the motor coupled to the compressor due to the torque ripple of the compressor. In FIG. 3A, θ denotes a rotational angle of the motor. Although the detailed structure of an apparatus for controlling a rotational speed of the motor installed in the compressor may be changed according to a kind of the motor, the apparatus generally includes a speed controller and a current controller. In addition, when the apparatus for controlling the rotational speed of the motor estimates the speed of the motor, the apparatus detects current and voltage values supplied to the motor, and estimates the speed of the motor on the basis of the detected current and voltage values. Here, the apparatus for controlling the rotational speed of the motor has the characteristics (torque ripple) of the compressor as shown in FIG. 3A, and thus generates the speed ripple of the motor as shown in FIG. 3B. However, vibrations and noises of the compressor are increased and performance thereof is decreased due to the speed ripple of the motor.
On the other hand, in order to solve the above problems, a conventional method for compensating for a torque calculates load characteristics (compressor torque) in advance in an offline method, and generates a lookup table to control the motor torque according to the previously-calculated load characteristics. The conventional method for compensating for the torque will now be explained with reference to FIG. 4.
FIG. 4 is a graph showing the conventional method for compensating for the torque. The conventional method for compensating for the torque is generally used in a brushless DC (BLDC) motor which does not supply a pulse width modulation (PWM) signal in electrical 60° rotational section and which supplies the PWM signal only in 120° rotational section. According to sensorless characteristics of the BLDC motor driven by measuring a counter-electromotive force in 60° rotational section, a position of a rotor can be sensed merely at electrical 60° interval. Therefore, the position of the rotor is detected 6 times in one electrical rotation, and the torque of the motor is compensated for on the basis of the detected position values of the rotor.
However, as depicted in FIG. 4, according to the method using the lookup table for storing the previously-calculated compensation value, the torque of the motor can be compensated for merely in the position of the rotor where the compensation value stored in the lookup table has been calculated. Accordingly, a reference position of the rotor needs to be set. Here, an error of the reference position of the rotor has a reverse effect on compensation for the torque.
In the case that the compensation value corresponding to the lookup table is used, compensation values considering load variations in the compressor must be all experimentally calculated. Moreover, when a model of the compressor is upgraded, compensation values considering load variations in the compressor must be all experimentally calculated in advance.
On the other hand, the conventional apparatus for controlling the rotational speed of the motor was disclosed under U.S. Pat. No. 6,646,409 on Nov. 11, 2003.